You've had heaps of awesome advice, but I wanted to say that my left boob can be sooooo slow with the let down that ds (and dd used to too) fusses and cries and carries on. I just switch them to the right one till I feel a let down and then switch back to the left (I'm talking up to five minutes for a let down if I'm feeling stressed.)

Good luck! Also rest and sleep as much as you can. Mastitis is nasty and takes a while to recover from.